The present invention relates to in situ measurements of earth formations traversed by a well borehole. More particularly, the invention relates to pulsed neutron irradiation measurement techniques for in situ differentiation of low porosity limestones from high porosity gas sands.
Pulsed (d,t) sources used in borehole logging produce neutrons which have energies of 14 Mev. These neutrons, when emitted into the borehole, are then moderated by interaction with the nuclei of the materials in the borehole and the surrounding earth formations as the diffuse therethrough. When the neutron energies have moderated to below about 0.05 electron volts, they come into thermal equilibrium with their environment. After reaching this thermal energy range, the neutrons continue diffusing through the formation and borehole until they are captured by nuclei in the constituent materials. The rate at which a zone of material (formation or borehole) captures these thermal neutrons (or more precisely, the probability of capture) is referred to as the macroscopic capture cross-section of the zone. The macroscopic capture cross-section is, in turn, a result of the combined microscopic capture cross-sections of the various constituent elements and materials constituting the zone. The capture cross-section of chlorine for thermal neutrons is considerably higher than that of most other elements commonly encountered in earth formations of interest. Accordingly, thermal neutron macroscopic capture cross-section measurements can give a good indication of the saline content of the fluids in the zone in question. By combining such information about the saline content of the fluids in the pore spaces of an adjacent earth formation with information about the formation water salinity, porosity measurements, and measurements of formation shaliness, information can be derived which can discriminate oil from salt water filled pore spaces in the vicinity of a well borehole.
Since thermal neutrons are absorbed by other materials as well as chlorine, the macroscopic capture cross-section is also responsive to borehole conditions and to the lithology of the formation materials. Prior art thermal neutron cross section methods have therefore typically been structured to try to minimize these effects. Borehole effects have been minimized, for example, by delaying the measurements after each neutron pulse so that these effects could then be ignored, since borehole moderation and die away is usually faster than formation moderation and die away.
U.S. Pat. No. 4,409,481 (Smith, Jr. et al., issued Oct. 11, 1983) and U.S. Pat. No. 4,424,444 (Smith, Jr. et al, issued Jan. 3, 1984), both assigned to the assignee of the present invention, disclose important improvements in such thermal neutron measurements. In these inventions, at least four, and preferably six, capture gamma ray count rate measurements are made starting immediately after thermalization of the fast neutrons. The logging systems which are disclosed in these patents are designed to measure .SIGMA..sub.FM, the thermal neutron capture cross section of the formation, and .SIGMA..sub.BH, the borehole capture cross section. As with prior pulsed neutron systems, a 14 MeV pulsed neutron generator source is used to create a time dependent thermal neutron, and hence capture gamma ray, distribution in the vicinity of two gamma ray detectors within the logging tool. The decay rate of the capture gamma radiation measured by the tool is used to obtain .SIGMA..sub.FM, .SIGMA..sub.BH, and also a number of other parameters useful in evaluating log quality, borehole conditions, and reservoir performance. Reference should also made to the following publications wherein additional aspects of these inventions are discussed: Smith, H. D., Jr., Arnold, D. M., and Peelman, H. E., "Applications of a New Borehole Corrected Pulsed Neutron Capture Logging System (TMD)", Paper DD, SPWLA Twenty Fourth Logging Symposium Transactions, June 1983; and Buchanan, J. C., Clearman, D. K., Heidbrink, L. J., and Smith, H. D., Jr., "Applications of TMD Pulsed Neutron Logs in Unusal Downhole Logging Environments", Paper KKK, SPWLA Twenty Fifth Logging Symposium Transactions, June 1984.
An important practical problem which occurs in neutron logging using dual detector thermal neutron decay measurements is the differentiation of tight formations (i.e., low porosity) from high porosity gas formations: both have very similar neutron parameters (low ratio porosity and low sigma values). It turns out that along the Gulf Coast (and similar sand/shale sequences) naturally occurring low porosity formations are almost always limestones, not sandstones. Hence, a practical means for differentiating high porosity gas sands from low porosity hard streaks would be to determine the relative amounts of sand and limestone present in the formation. Formations that have low ratio porosity and low sigma values simultaneously with a low amount of sand and a high amount of limestone would be rejected, whereas formations with a large amount of sand and a small amount of limestone may be identified as high porosity gas sands of considerable commercial interest. (In the past, such zones have often been overlooked in comparison with the original target, or have developed as a result of production practices.)
Unfortunately, present dual detector thermal neutron decay measurement tools usually cannot make the above differentiation because these formations have similar hydrogen indices and the porosity response of such tools is strongly influenced by the formation hydrogen index.
A need therefore remains for a method and apparatus for extending the operation of such dual detector thermal neutron decay measurement tools to include a relative sandtone versus limestone measurement. Such a method and apparatus should provide a sensitive and accurate indication and means for differentiating between such formations, should be versatile and reliable, and readily suited to use on the widest range of such thermal neutron decay measurement tools.